(1) Field of the Invention
The present invention relates to a two-component type magnetic developer comprising a toner and a resin-coated magnetic carrier.
(2) Description of the Related Art
In the field of commercial electronic reproduction, there is widely used the two-component type magnetic developer comprising a toner and a magnetic carrier for developing an electrostatic image formed on the surface of the photosensitive material.
An ordinary developing mechanism in which a developer as described above is used has a structure as shown in FIG. 1. More specifically, a box-shaped toner supply mechanism 4 is arranged on the developing mechanism 2 and a toner 6 is supplied from above. The toner 6 is fed into a developing device 10 disposed below through a supply opening 8 equipped with a feeder and is stirred together with a carrier in the developing device 10 by stirrers 12 to form a two-component type developer 14.
A developing sleeve (developer-supporting member) 16 equipped with many magnetic poles is arranged in the developing device 10. The developer 14 having the frictionally charged toner is supplied into the developing sleeve 16 and a magnetic brush 18 of the developer is formed on the surface of the sleeve by a magnetic force. The length of the magnetic brush 18 is adjusted by a brush-cutting mechanism 20, and a uniform layer of the developer is formed on the surface of the developing sleeve 16. This developer layer is delivered to the nip position to a surface photosensitive layer 24 of an electrophotographic photosensitive material drum (image carrier) 22. The photosensitive material drum 22 is arranged apart by a distance DD-S from the developing sleeve 16, and the developing sleeve 16 and photosensitive material 22 are rotatably supported and are driven so that the moving directions (indicated by arrows) of the sleeve 16 and drum 22 are the same at the nip position (the rotation directions are reverse to each other).
A corona charger 26 connected to a variable high voltage power source 25 and an optical system 28 for the light exposure are arranged around the photosensitive material drum 22 upstream of the developing device 10 to form an electrostatic latent image having a predetermined surface voltage. A bias power source 33 equipped with a voltage-adjusting mechanism 30 is connected between the photosensitive drum 22 and the developing sleeve 12 so that an optional value voltage (bias voltage) which has the same polarity as that of the surface voltage and is lower than the surface voltage is applied onto the photosensitive layer 24. A transfer mechanism 34 for transferring a toner image to a copying machine is arranged around the photosensitive layer 24 downstream of the developing zone.
In the above-mentioned structure, the developer 14 forms the magnetic brush 18 on the developing sleeve 16 and at the nip position, this magnetic brush 18 reacts with the electrostatic latent image of the photosensitive layer 24 to form a visible image of the toner on the photosensitive layer 24.
As a magnetic carrier in the two-component type developer, there is widely used a resin-coated magnetic carrier of which the surface is coated with an acrylic resin, etc. for preventing adhesion of the toner to the surface of the carrier, so-called "spent toner". In such resin-coated magnetic carrier, the deterioration of the carrier caused by the surface oxidation is effectively prevented. Accordingly the resin-coated magnetic carrier has an advantage that contamination of the toner (for example, discoloration of the toner) caused by oxides on the carrier surface can be prevented. As a consequence of the foregoing, such resin-coated magnetic carrier is effectively used in combination with a color toner, such as a white toner.
However, in case where a white toner, etc. compounded with a coloring agent having a low electroconductivity, such as titanium oxide, is used as the toner component of a two-component type magnetic developer, the charge quantity of the toner becomes very large compared with the charge quantity of the electrostatic latent image formed on the surface of the photosensitive layer upon development as a consequence of a high electric resistance of said toner. As a result, the obtained image density is low. That is to say, the decrease in the image density is caused by the presence of too few toner particles which have charge quantity sufficient to balance with the charge quantity of the electrostatic latent image.
Thus, as a means to avoid such decrease in the image density, it is conceivable to decrease the mount of resin coated on the magnetic carrier for the purpose of decreasing the electric resistance of the carrier. However, in case where acrylic resin is used as the coating resin, the amount of coating has to be decreased to a far extent in order to sufficiently decrease the electric resistance of the carrier. With such extreme decrease in the amount of coating, the resin film formed on the surface of the carrier becomes so thin as will render it impossible to effectively protect the surface of the carrier.